Carrying out the mapping of a zone of terrain consists mainly in pinpointing the geographical position of the prominent elements located over the zone considered, relief elements or fixed objects in particular. This mapping is generally carried out by means of detection systems mounted on an appropriate vehicle, the vehicle traversing the ground of the zone considered, or overflying this zone, as the case may be. In practice, the detection system determines the relative position of the prominent elements with respect to the vehicle and the absolute position of each element is determined by associating the measured relative position and the position, assumed known, of the vehicle at the instant of detection and which is for example determined by the navigation system of the vehicle.
Thus for example the mapping of an emerged zone of terrain may be carried out using a radar system having sufficient resolution, this radar being mounted on an aircraft, a remotely controlled aircraft or else an automatic system of Drone type. Hence the aircraft is deployed above the zone considered and performs the determination of the position of each prominent element by means of the measurements performed by the radar, which give the relative position of the element considered with respect to the aircraft and information about the geographical position of the aircraft, a position generally determined with the aid of a system of GPS receiver type located aboard the aircraft. Hence the position of the aircraft being known with precision, the position of the element considered may be determined with great precision, as long as the measurements provided by the radar have the desired precision.
However, there exist circumstances where the determination of the position of the prominent elements cannot be carried out in this way with sufficient precision.
Such is notably the case if the determination of the relative position of the prominent elements with respect to the vehicle tasked with carrying out the mapping is not carried out with the desired precision; because the measurements carried out by the detection system do not have the desired precision for example.
Such is also the case if the geographical position of the vehicle is known with insufficient precision. Such is for example the case if the vehicle is an aircraft which does not have any GPS system. Such is also the case if, for example, the zone to be mapped is a submerged, underwater zone. The mapping is then carried out with the aid of a sonar system carried by an autonomous or non-autonomous underwater vehicle, which cannot possibly determine its position with the aid of the GPS system, GPS information being, in a known manner, inaccessible to a vehicle being deployed under water.
In these last two cases, the determination of the position of the vehicle at each instant of its displacement is carried out by implementing conventional means, inertial means for example, which, commencing from a starting position, assumed to be known with precision, determine the relative displacement of the vehicle with respect to this origin.
This so-called dead reckoning navigation technique makes it possible to estimate the position of the vehicle at any instant. The measurements carried out with the aid of such means are then generally less precise. Moreover a drift is noted in the course of time of the determination of the absolute position of the vehicle with respect to its real position. Ultimately the absolute position of the prominent elements is estimated with lesser precision, a precision which is sometimes even insufficient.
As regards the mapping of an underwater zone, the latter is generally carried out by means of an underwater vehicle, an underwater drone for example, equipped with a lateral sonar and being deployed above the zone to be mapped, in proximity to the bed. The position measurements are generally carried out by grazing insonification of the seabed.
This type of insonification advantageously makes it possible to chart a prominent object not by the echo that it reflects but by the “acoustic” shadow that it casts on the bed. Detection based on acoustic shadow is particularly advantageous for charting certain objects, certain stealthy underwater mines in particular, which hardly reflect, if at all, the sound wave emitted by the sonar but which nonetheless act as a screen and therefore produce an acoustic shadow. It is recalled here that the acoustic shadow cast by an object can be defined here by the bed zone for which the object considered constitutes a screen preventing its insonification.
Hence by implementing any known appropriate processing, it is possible to determine the contours of the acoustic shadow cast by a prominent object, which contours make it possible to determine the position and the profile of the object itself and to carry out a classification of the located objects, in an easier manner than on the basis of the echoes reflected by the objects themselves, notably if this classification is carried out in an automatic manner.
However the determination of the exact position of an object, on the basis of the acoustic shadow that it produces, is sometimes difficult and in any event approximate. It depends in particular on the angle of insonification and the direction in which the object is insonified. Hence, even if the absolute position of the vehicle is known at any instant with precision, the relative position of the object with respect to the vehicle, and consequently its absolute position, can only be determined, estimated, with a bias due to the shift between the position of the acoustic shadow and that of the real object.
Furthermore, the zone to be mapped being insonified from various directions, on account of the displacements of the vehicle above this zone, it sometimes happens that one and the same prominent object is insonified several times in different directions. It then produces distinct acoustic shadows which give rise to the determination, for one and the same object, of several detections having different estimated positions, and which leads to several observations being identified for a single real object.
Hence it is then necessary to refine the mapping carried out by implementing appropriate means for associating the observations so as to determine whether two localized characteristic elements do or do not constitute one and the same element considered from different angles.
In the zones of low or mean density, the known solutions successfully carry out the pairing of the observations relating to one and the same object. Hence, the position of an object having formed the subject of multiple locations, these locations having been recognized as relating to this same object, can then be re-estimated with greater precision by techniques for calculating weighted averages.
On the other hand, no particular benefit is derived from this readjustment operation as regards the other prominent objects which have formed the subject of only a single detection. The resulting mapping therefore makes it possible only to fuse the observations representing one and the same object, and thus to improve the precision of the estimation of the position of this object. It does not make it possible to improve the global precision of the location of the other objects detected in the mapped zone.
Moreover, these known solutions are not concerned with the problem posed by the positioning errors pursuant to the vehicle's absolute positioning error, which error is due mainly to the precision and to the drift of the navigation system which estimates the absolute position of the vehicle.